A large number of integrated circuits are currently used in electronic apparatuses. Most of the integrated circuits that are used in the electronic apparatuses are application specific integrated circuits (ASICs). The application specific integrated circuit is a dedicated circuit designed for a specific electronic apparatus. The circuit configuration of the application specific integrated circuit, including the arrangement of logic cells (unit logic circuits for AND circuits, OR circuits, or the like) and connections among the logic cells, cannot be changed after manufacturing.
In recent years, competition for development of electronic apparatuses has been intensifying, and much effort has been made to miniaturize the electronic apparatuses. In these circumstances, much attention has been paid to programmable logic ICs (rewritable logic integrated circuits) which, even after manufacturing, allow specific functions from among many functions on one chip to be selected by changing the circuit configuration with electronic signals. In the programmable logic IC, a plurality of logic cells are connected together via switches. Typical examples of the programmable logic IC include an FPGA (Field-Programmable Gate Array) and a DRP (Dynamically Reconfigurable Processor).
In spite of much attention being paid to the programmable logic IC, the number of examples in which the programmable logic IC is mounted in the electronic apparatus has been limited. The reason is as follows. In the conventional programmable logic IC, switches connecting logic cells together have a large size and thus a high ON-resistance. Thus, a configuration having a small number of logic cells each including a large number of transistors has been adopted in order to minimize the number of switches installed. This reduces the degree of freedom of the combination of the logic cells, limiting the functions of the programmable logic IC. That is, the large size and high on resistance of the switch limit the functions of the programmable logic IC. The mounting of the programmable logic IC in the electronic apparatus is thus limited.
Thus, the size and ON-resistance of the switch, connecting the logic cells together, need to be reduced in order to provide the programmable logic IC with various functions to promote the mounting of the programmable logic IC in the electronic apparatus and the like. As a switch that meets this demand, a switching device has been proposed which utilizes metal ion migration and electrochemical reaction in an ion conductor (a solid through which ions can migrate freely) (see, for example, WO 2003/094227). The switching device disclosed in WO 2003/094227 has a smaller size and a lower ON-resistance than semiconductor switches (MOSFETs) often used in the conventional programmable logic ICs.
FIG. 7 is a schematic sectional view showing the configuration of the switching device disclosed in WO 2003/094227. The switching device has first electrode 11, and second electrode 12 laminated on first electrode 11 via ion conductive layer 13 (referred to as a “solid electrolyte” in Patent Document 1). In this case, ion conductive layer 13 serves as a medium through which metal ions are conducted.
Now, the operation of the switching device shown in FIG. 7 will be described. Second electrode 12 is grounded, and a negative voltage is applied to first electrode 11. Then, metal in second electrode 12 is dissolved into ion conductive layer 13 as metal ions. The metal ions in ion conductive layer 13 are precipitated on a surface of first electrode 11 as metal. The precipitated metal forms metal dendrite connecting first electrode 11 and second electrode 12. The metal dendrite is a metal precipitate resulting from precipitation of the metal ions contained in ion conductive layer 13. The metal dendrite electrically connects first electrode 11 and second electrode 12 together to turn on the switch.
On the other hand, with the switch in the on state, second electrode 12 is grounded, a positive voltage is applied to first electrode 11. Then, the metal dendrite is dissolved into ion conductive layer 13, and a part of the metal dendrite is severed. This electrically disconnects first electrode 11 from second electrode 12 to turn off the switch. Electrical characteristics change before the electric connection is completely cut. For example, the electric resistance between first electrode 11 and second electrode 12 increases or inter-electrode capacitance changes before the electric connection is finally cut. Furthermore, a desirable material for first electrode 11 is a material which does not feed metal ions into the ion conductive layer when the voltage is applied to first electrode 11. Additionally, the negative voltage may be applied to first electrode 11 again in order to change the off state to the on state.
JOURNAL OF SOLID STATE CIRCUITS, Vol. 40, No. 1, 2005, pp. 168 to 176 proposes that such a switching device as shown in FIG. 7 be used as a wiring switch for the programmable device. Compared to conventional switches, this switching device reduces the switch area to 1/30 and the switch resistance to 1/50. Moreover, the switching device can be formed into a wiring layer. This is expected to reduce the chip area and to improve possible wiring delays. Furthermore, the size of the logic cells in the programmable logic IC can be reduced, allowing a drastic increase in circuit utilization efficiency. As a result, the chip area is reduced to 1/10, and power efficiency is tripled. The large chip size and low power efficiency of the conventional programmable logic IC limits the range of applications thereof. However, programmable logic ICs using such a switching device as shown in FIG. 7 can cover a wider application range.
WO 2003/094227 discloses Cu/Cu2S, Ag/Ag2S, and the like as examples of a combination of an electrode material and an ion conductive layer. Any of the material combinations involves the application of a voltage (switching voltage) of about 0.05 to 0.30 [V] to first electrode 11 (FIG. 7) in order to change the switching device from the on state to the off state or from the off state to the on state. On the other hand, for logic signals used in the programmable logic IC as signals, a voltage indicating one of two types of information, that is, an operating voltage for the logic IC, is Vdd [V]. A voltage indicating the other type of information is 0.0 [V]. A Vdd [V] that is currently often used in silicon integrated circuits is about 1.0 to 2.0 [V].
The switching device disclosed in WO 2003/094227 has a switching voltage of at most 0.30 [V]. Consequently, if the Vdd of the logic signal is 1.0 [V], the switching voltage is lower than Vdd. Thus, every time a logic signal with the voltage Vdd [V] is input to the switching device, a voltage of 1.0 [V] is applied to the first electrode. The logic signal may thus change the state of the switch. In this case, a fatal problem may occur; the switch may not function. Therefore, a higher switching voltage needs to be set to stabilize the switching device.
Furthermore, a time equal to or longer than the product life (generally 10 years) of the programmable logic IC is required to hold the state of the switching device (the time required to maintain non-volatility). Heat energy at room temperature is generally 26.0 [meV]. Thus, when the switching voltage is closer to 26.0 [mV], thermal noise may be generated and is likely to cause the switching state to change voluntarily. Therefore, the switching voltage also needs to be increased in order to allow the state of the switching device to be maintained for a longer time.